The technique for cellar-type mobile communication systems has been shifted from the Universal mobile telecommunication system (UMTS) to the long term evolution (LTE). The LTE adopts the orthogonal frequency division multiplexing (OFDM) and single carrier-frequency division multiple access (SC-FDMA) techniques for downlink and uplink wireless accesses, respectively, achieving high-speed wireless packet communications with a downlink peak transmission rate of 100 Mb/s or higher and an uplink peak transmission rate of 50 Mb/s or higher. The 3rd Generation Partnership Project (3GPP), an international standardization organization, has initiated a study on the mobile communication system LTE-A (LTE-Advanced) based on the LTE, for achieving further faster communications. The LTE-A is aiming at a downlink peak transmission rate of 1 Gb/s and an uplink peak transmission rate of 500 Mb/s, and a various new techniques have been reviewed for a radio access scheme, a network architecture, and the like (Non-patent Reference 1).
The 3GPP is currently studying the coordinated multi-point (CoMP) communication scheme for the LTE-A. For downlink CoMP communications, techniques, such as a joint transmission, a coordinated beamforming, a coordinated scheduling, and a fast cell selection, have been considered.
Hereinafter, the basic concepts of the joint transmission, the coordinated beamforming, and the coordinated scheduling will be described with reference to FIG. 1. In the following description, the term “cell” is used to mean each geological range where a wireless base station provides a wireless service, and it may also be used to mean a part of the communication function managed by the wireless base station for communicating with user equipments in the each geological range. In FIG. 1, a serving cell is a sell that exchanges control information with a mobile station UE. A coordinated cell is a cell that communicates with a mobile station UE, together with the serving cell, using the CoMP communication technique (i.e., provides coordinated communications). Although coordinated transmissions are provided by the two cells in the example in FIG. 1, generally there may be three or more coordinated cells.
FIG. 1(a) schematically depicts the joint transmission by two cells, as one example.
In the joint transmission, identical data is sent to a single user equipment UE simultaneously from multiple cells using the same radio resource. In the example in FIG. 1(a), the mobile station UE measures the characteristics of the downlink propagation paths from the serving and coordinated cells, and reports the measurement results to the serving cell. The serving cell and the coordinated cell execute precoding and scheduling based on information that is fed back to them, and cooperatively send the identical data to the mobile station UE. Since the mobile station UE receives a synthetic signal from signals from the serving and coordinated cells, the reception SINR is improved as compared to the case where the signal is received only from the serving cell. The transmission data, propagation path information, scheduling information, precoding setting information, and the like are shared among the cells that cooperate with each other. As an interface among base stations for sharing the information, the LTE stipulates the X2 interface (Non-patent Reference 2).
FIG. 1(b) schematically depicts the coordinated beamforming and the coordinated scheduling by two cells, as one example.
In the coordinated beamforming and the coordinated scheduling, data is sent to different mobile stations (user equipments UE1 and UE2 in the example in FIG. 1(b)) in coordinated cells coordinated, and precoding or scheduling is cooperatively controlled. Precoding is predominantly cooperatively controlled in cooperating cells in the coordinated beamforming, while scheduling is predominantly cooperatively controlled in cooperating cells in the coordinated scheduling. Unlike the joint transmission, transmission data may not be shared among cooperating cells in the coordinated beamforming and the coordinated scheduling. In the example in FIG. 1(b), a serving cell sends data to the user equipment UE1, and the coordinated cell sends the data to the user equipment UE2. Here, precoding or scheduling is cooperatively controlled among the cells such that the interference waves from the serving cell to the user equipment UE2 and the interference waves from the coordinated cell to the user equipment UE1 are reduced. Hence, communications in the coordinated beamforming and the coordinated scheduling is improved compared to the cases where each cell communicates with the user equipment without any cooperation. In the coordinated beamforming and the coordinated scheduling, propagation path information, scheduling information, and the like are shared among cooperating cells.
Non-patent Reference 1: 3GPP TR 36.912 v9.1.0    Non-patent Reference 2: 3GPP TS 36.423 v9.1.0